A POLYKETIDE SYNTHASE IS REQUIRED FOR FUNGAL VIRULENCE AND PRODUCTIONOF THE POLYKETIDE T-TOXIN

Race T of the fungal pathogen Cochliobolus heterostrophus is highly vi rulent toward Texas male sterile (T) maize and differs from its relati ve, race O, at a locus (Tox1) that is responsible for the production o f T-toxin, a family of linear long-chain (C-35 to C-41) polyketides. I n a previous study, the restriction enzyme-mediated integration proced ure was used to mutagenize and tag Tox1, Here, we report that the DNA recovered from the insertion site of one mutant encodes a 7.6-kd, open reading frame (2530 amino acids) that identifies a multifunctional po lyketide synthase (PKS)-encoding gene (PKS1) with six catalytic domain s arranged in the following order, starting at the N terminus: beta-ke toacyl synthase, acyltransferase, dehydratase, enoyl reductase, beta-k etoacyl reductase, and acyl carrier protein. PKS1 is interrupted by fo ur apparent introns (74, 57, 49, and 41 bp) and exists in the genome a s a single copy surrounded by highly repetitive, A+T-rich DNA. When PK S1 in race T was inactivated by targeted gene disruption, T-toxin prod uction and high virulence were eliminated, indicating that this PKS is required for fungal virulence. Race O strains, which do not produce T -toxin, lack a detectable homolog of PKS1, suggesting that race T may have acquired PKS1 by horizontal transfer of DNA rather than by vertic al inheritance from an ancestral strain.